Microlithography is used for producing microstructured components, such as integrated circuits or LCDs, for example. The microlithography process is carried out in a so-called projection exposure apparatus comprising an illumination device and a projection lens. The image of a mask (reticle) illuminated by the illumination device is in this case projected by the projection lens onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In projection lenses designed for the extreme ultraviolet (EUV) range, i.e. at wavelengths of e.g. approximately 13 nm or approximately 7 nm, owing to the lack of availability of suitable light-transmissive refractive materials, mirrors are used as optical components for the imaging process.
In the illumination device of a microlithographic projection exposure apparatus designed for operation in the EUV range, in particular the use of facet mirrors in the form of field facet mirrors and pupil facet mirrors as focusing components is known for example from DE 10 2008 009 600 A1. Such facet mirrors are constructed from a multiplicity of individual mirrors or mirror facets, which may be designed to each be tiltable by way of flexure bearings for the purposes of adjusting, or else for realizing, specific illumination angle distributions. These mirror facets may comprise a plurality of micromirrors in turn.
Moreover, the use of mirror arrangements which comprise a multiplicity of mutually independently adjustable mirror elements in an illumination device of a microlithographic projection exposure apparatus, designed for operation at wavelengths in the very ultraviolet (VUV) range, for adjusting defined illumination settings (i.e. intensity distributions in a pupil plane of the illumination device) is also known, for example, from WO 2005/026843 A2.
In practice, there is a need during the production of mirror elements to adjust the respective refractive power thereof as exactly as possible, wherein this may be a refractive power of zero (corresponding to a plane mirror element) or else a refractive power differing from zero, depending on the application. A known approach to this end consists of using the mechanical tension generated when applying a layer stack, including the reflection layer system, onto a substrate and the bending force exerted on the substrate by the layer stack as a result thereof in a targeted manner when manufacturing the respective mirror element in order to generate a setpoint curvature of the mirror element—and hence a desired finite refractive power of the mirror element (wherein the substrate has a curvature deviating from the setpoint curvature of the mirror element prior to the formation of the layer stack).
A problem occurring in practice is that the mirror elements are exposed to temperature changes (both during the commissioning and during the subsequent running operation of the respective optical system). In the case of differing thermal expansions of the layer stack on the one hand and the substrate on the other hand (i.e. as a consequence of the so-called bimetallic effect), this produces an unwanted change in the curvature or refractive power of the respective mirror element and consequently may lead to a deterioration in the optical properties of the optical system comprising the mirror element.
Regarding the prior art, reference is made merely by way of example to WO 2015/114043 A1, DE 10 2010 028 488 A1 and DE 10 2006 057 567 A1.